The present invention concerns spinal fixation systems, and particularly spinal hooks for engaging portions of a vertebra. More specifically, the present invention concerns improvements in the manner of connecting the spinal hook to a rod used in the spinal fixation system.
Several techniques and systems have been developed for correcting and stabilizing spinal curves and facilitating spinal fusion. In one system, a bendable rod is longitudinally disposed adjacent the vertebral column and is fixed to various vertebra along the length of the column by way of a number of fixation elements. A variety of fixation elements can be provided which are configured to engage specific portions of the vertebra.
The present invention specifically concerns one type of fixation element, that is a spinal compression/distraction hook of the type used in connection with the bendable rod. The spinal hooks are used to anchor the rod relative to the spinal column, as well as to provide a path for compression or distraction loading of the vertebra.
An example of one such system is the TSRH.TM. spinal system of Danek Medical, Inc. In this system, a spinal hook, such as hook H shown in FIG. 1, is engaged to the fixation rod R by way of an eyebolt assembly E. As is known in the art, the eyebolt E is threaded onto the spinal rod and captured within yokes on the spinal hook. A nut is threaded onto the threaded post of the eyebolt to clamp the yoke between the nut and the fixation rod R. The eyebolt E and hook yokes provide three degrees of fixation as represented by the arrows in FIG. 1. Details of the TSRH.TM. spinal implant system are disclosed in the "TSRH.TM. Surgical Technique Manual" provided by Danek Medical, Inc. published in 1990 which disclosure is incorporated herein by reference.
It is the goal of the surgeon using such spinal implant systems as the Danek TSRH.TM. system to apply the vertebral fixation elements, such as the spinal hook, to the spine in the appropriate anatomic position, and then to engage each fixation element to the spinal rod. It has been found that in spite of the great benefits provided by the double-yoke spinal hooks, such as hook H in FIG. 1, certain problems arise which have heretofore not been adequately addressed.
One problem with the spinal hooks H of the prior art is that the hooks are rather bulky and wide since the fixation yokes of the hook are configured to surround the spinal rod R. The size or bulkiness of the spinal hook is an especially important problem since a significant portion of the patient population for spinal implant systems of this type (namely the TSRH.TM. system) is made up of pediatric patients. Bulky implants are not easily implanted into small, thin people because the space around the vertebra is not great.
In patients with severe deformities of the spinal column, the rod must be contoured to meet the deformity. Prior art hooks, such as hook H, only allow the rod to be implanted in one position relative to the spinal column as dictated by the required position of the blade of the spinal hook. There is therefore a need for a spinal hook that can be fixed in more than one position on the rod, thereby allowing the rod to be implanted in more than one position relative to the spinal column.
Another problem with the spinal hooks H of the prior art is that the instrumentation used to engage the hook and hold it for insertion is often difficult to engage to the hook and can be susceptible to a fragile engagement between the instrumentation and the hook. For example, in one prior art embodiment of the spinal hook H, pin holes P are situated in each of the four posts of the double-yoke hook. The pin holes P are designed so that they receive the pins of a hook holder. Such hook-holding instruments incorporate four or eight pins to correspond to the number of pinholes in the hook (which in the case of the hook H in FIG. 1 is 8 pinholes). Engagement of the pins to the pinholes is often difficult as all the components must be aligned perfectly for the instrument to be locked into place. Moreover, as the hook-holding instrument is manipulated to properly position the spinal hook about a lamina L (as shown in FIG. 2), there is a risk that the pins will become disengaged with the pinholes P. It has been found that these frailties of the present hooks H as shown in FIGS. 1 and 2 can often lead to significant amounts of manipulation in order to properly implant the hook.
As shown in FIG. 2, the placement of the spinal hook H and engagement of the hook to the rod R by way of the eyebolt assembly E must frequently occur in very tight quarters. In one particular application shown in FIG. 2, the hook is oriented between the L2 and L3 lumbar vertebra so that the hook engages the lamina L of the L2 vertebra. It can certainly be appreciated that in smaller patients, such as pediatric patients, the room to manipulate the spinal hook to engage it to the rod is very limited.
There is therefore a need for a new spinal hook configuration which is less bulky and easier to engage than the spinal hooks previously known. There is also a need for such a spinal hook which can be more easily engaged to the spinal rod, preferably on either side of the rod as required by the spinal anatomy.
A new spinal hook is offered by the present invention which has a smaller profile and which includes means for more easily engaging a hook holder instrument for implanting the hook. More particularly, and in one aspect of the invention, the spinal hook comprises a hook-configured shoe to which a central post top is engaged. The top portion includes a pair of central posts which are displaced to form a slot therebetween. Each central post includes a groove colinearly formed to receive a spinal rod within. The slot between the two posts is oriented with respect to the grooves so that an eyebolt assembly may be used to engage the hook to the fixation rod.
In another aspect of the invention, a slot is formed in the end faces of the hook. The slot forms one part of a tongue and slot instrument engagement arrangement, with the hook-holding instrument including a correspondingly configured tongue. The tongue of the hook-holder instrument engages within the slot on each central post of the hook to provide a more substantial engagement as the hook is being implanted.
In another aspect of the invention, each of the pair of central posts include grooves on both lateral faces of the posts so that the spinal rod can be situated on either side of the spinal hook. Thus, the spinal hook of the present invention is more readily adaptable to various spinal anatomies without requiring that the spinal rod be bent in the saggital plane in order to contact the spinal hook.
It is one object of the spinal hook of the present invention to reduce the overall profile of the spinal fixation system, and particularly the components used to engage the vertebra of the patient.
It is another object to provide such a hook which incorporates a tongue and slot arrangement for a firm, solid and easy engagement to a hook-holding instrument.
Related objects and advantages of the present invention will be apparent from the following description.